The specific aims of the proposed investigation to study human immunity to type III, group B streptococcal infection, would be: 1) to use the type III capsular polysaccharide antigen of group B streptococci isolated and purified in large molecular weight form in our initial studies in order to develop simple and sensitive serologic assays for the detection of anti-type III, group B streptococcal antibodies in animals and man; 2) to analyze the age-related and disease-related serologic respone to type III strains in normal individuals, asymptomatic carriers (infants, pregnant and non-pregnant women, and adult men) and in patients with significant infections due to type III strains; 3) to determine the protective activity of the purified type III polysaccharide antigen in animal models for type III, group B streptococcal meningitis; and 4) to collect epidemiologic and immunologic data in order to identify a group of babies at high risk to the development of serious type III infections in order to determine the potential feasibility of a type III, group B streptococcal polysaccharide vaccine in pregnant women as a means of preventing symptomatic infection in their neonates and infants.